This invention relates to a photographic element for use in a process for forming an image, for example, through heat development, and more particularly, to such a photographic element having an electroconductive layer.
Heat developable photosensitive materials and heat development process are well known in the art and described in the literature, inter alia, "Fundamentals of Photographic Engineering", Corona Publishing K.K., Tokyo, Japan (1979), pages 553-555; "Image Information", April 1978, page 40; Nebletts Handbook of Photography and Reprography, 7th ed., Van Nostrand Reinhold Company, pages 32-33; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, and 3,457,075; British Patent Nos. 1,131,108 and 1,167,777; and Research Disclosure, June 1978, pages 9-15 (RD-17029).
A process for transferring a mobile dye imagewise formed by heat development to an image-receiving layer by heating and image-receiving materials used therefor are described in Japanese Patent Application Kokai Nos. 58-58543, 58-79247, and 59-168439, inter alia.
Heating of these heat-developable photosensitive materials and image-receiving materials (both generally referred to as photographic materials, hereinafter) may be carried out by a number of methods including contacting of photographic materials with a heat block having a great capacity; direct heating of photographic materials by laser and infrared irradiation, ultrasonic heating, high frequency heating or the like; and passing of photographic materials through heated gas. These methods are, however, not successful in achieving satisfactory results because of their shortcomings. For instance, the use of a heat block is both time and power consuming to accomplish a uniform temperature distribution throughout the heat block, further, insufficient contact prevents smooth uniform heat transfer to the photographic material. The use of radiation like laser beam is disadvantageous in that a large sized apparatus is required or a compact system is difficult to incorporate. The use of heated gas is time consuming because the gas has essentially a low heat capacity.
To overcome these shortcomings, the use of an exothermic electroconductive layer in combination with a heat developable photosensitive layer was proposed as disclosed in, for example, U.S. Pat. No. 206,368 and Japanese Patent Application Kokai No. 48-66442.
Also, a variety of positive electroconductive layers (which herein designate electroconductive layers whose electric resistance increases with a temperature rise) intended for plane heaters were developed as disclosed in, for example, Japanese Patent Application Kokai Nos. 49-82734, 49-82735, 51-13991, 51-39742, 51-39743, and 52-87694.
These methods aim to prevent overheating by taking advantage of the nature of these materials in that their electric resistance increases with a temperature rise and have been applied as plane heaters and snow melting systems. It is possible to apply these positive electroconductive layer to photographic materials as described in, inter alia, Japanese Patent Application No. 58-229377.
Any desired choice of design may be made on an exothermic electroconductive layer so as to meet the intended application whether it is of the positive type as mentioned above, of the negative type wherein the electric resistance of an electroconductive layer decreases with a temperature rise, or of the neutral type wherein resistance does not depend on temperature. With respect to a binder used in such a layer, a choice may be made between hydrophilic binders and hydrophobic polymeric binders combined with organic solvents, depending on the desired properties of the electroconductive layer.
An attempt has been made to employ hydrophilic colloid as a binder in the electroconductive layer. However, electroconductive layers containing typical hydrophilic colloids, gelatin and polyvinyl alcohol tend to change their electric resistance when folded or bent. Such a change in electric resistance is undesirable because it can positively reveal itself as irregularities in a heat developed or heat transferred image. It is known that this phenomenon becomes outstanding particularly when the electroconductive material used is a carbon black having a dibutyl phthalate (DBP) absorption of at least 100 which is preferably used since it undergoes a little resistance change by ambient humidity. There is the need for further improvement in this respect.